Integrated circuits are common in electronic products. Electronic products often are comprised of integrated circuits interfaced to each other via a data bus or other data paths. Interface specifications for various digital logic families delineate voltage and current levels required for digital signals to be transferred between two or more integrated circuits. Interface specifications are utilized by integrated circuits through the use of output buffer circuits to drive a logical low or logical high signal across a data path. In addition, output buffer circuits are a way of interfacing different digital logic families of integrated circuits.
Typically, output buffer circuits often use an external voltage level, Vcc, as a source of the logic high level. Depending on the design, Vcc can range from 3.0 to 5.5 volts. Output buffer circuits generally use a system ground (GND) as a sink for a logic low output. Output buffer circuits generally use two complementary transistor devices. The first device is a p-channel pull-up metal-oxide semiconductor (MOS) transistor, whose source is connected to Vcc, and whose drain is connected to the output terminal. The second device is an n-channel pull-down MOS transistor, whose drain is connected to the output terminal, and whose source is connected to ground.
Generally, a MOS device, when the drain source voltage is greater than or equal to the difference between the threshold voltage and the gate source voltage, is in the saturation region and acts like a constant current source. The MOS device is in the linear region and acts like a resistor when the drain source voltage is less than the difference between the threshold voltage and the gate source voltage.
Using the MOS device switching characteristics, an input data signal controls each device at its gate via control logic. To output a logic high signal, the first pull-up device is turned on by the control logic, and the first pull-down device is turned off. Output switching to this high state allows current to flow from Vcc to the output terminal via the first device and provides for a high impedance state via the second device so that no output signal current may flow through it to GND. In order to output a logic low signal, the first pull-up device is turned off, thus providing for a high impedance state between Vcc and the output terminal. In this low state, no current will flow from Vcc to the output terminal. Concurrently, the first pull-down device is turned on, thus allowing current to pass from the output terminal to GND. Thus, the output buffer circuit acts as a sink for current, and the output signal is a logic low signal. Therefore, the transition of the signal at the output terminal from one state to another state requires switching one of the devices on while switching the other device off. The gates of the first pull up and first pull down device are typically coupled to receive a data signal having a logic level adapted to activate one of the devices and to deactivate the other.
One example of an application of an output buffer is in a memory system. A memory system is commonly used in products such as digital cameras, personal digital assistants, and video game systems. A typical memory system is used to store commands or data that will be used in conjunction with a microprocessor. With the development of faster and faster microprocessors, memory systems must also keep pace. Fast transition times are a factor in the design of increasing circuit speed. This is particularly true with respect to memory systems. However, the fast transition times are affected by ambient temperature. In semiconductor devices, it is common for the output buffer stages to diminish in current drive capacity in response to increases in temperature. Such reductions in current drive capacity can translate into reduced operating speeds as signal transitions in the output signals will be slower. Therefore, the output buffer circuit is slower and thus, system speed is slower as well.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative output buffer circuits and methods of their operation.